Charged particle accelerators



2 Sheets-Sheet 1 MENTOR? DOUGLAS PERCIVAL RAMSAY PEI'RIE & STANLEYERNEST HUNT $8M M! May 22, 1962 D. P. R. PETRIE ETAL CHARGED PARTICLEACCELERATORS Filed Nov. 9, 1959 D. P; R. PETRIE ETAL 3,036,233

CHARGED PARTICLE ACCELERATORS May 22, 1962 Filed Nov. 9. 1959 2Sheets-Sheet 2 DOUGLA$ PERQNAL- RAM$AY PETRIE.

ETANLEy E'QNEsT HUNT BY 5%! q! J k f 3,036,233 CHARGED PARTICLEACCELERATORS Douglas Percival Ramsay Petrie, Reading, and Stanley ErnestHunt, Caversham, Reading, England, assignors to MetropolitanickersElectrical Company Limited,

London, England, a company of Great Britain Filed Nov. 9, 1959, Ser. No.851,758 Claims priority, application Great Britain Nov. 11, 1958 3Claims. (Cl. 313-83) This invention relates to charged particleaccelerators.

The invention is applicable both to accelerators for positively chargedparticles and also to accelerators for negatively charged particles.

Charged particle accelerators such, for example, as Van de Graaffmachines, Cockcroft-Walton machines, accelerate particles by allowingthem to fall through a high potential diflerence between the ends of anaccelerator tube, the inside of which is maintained under conditions ofhigh vacuum. The tube is commonly sub-divided along its length byelectrodes maintained at potentials intermediate between those existingat the ends of the tube. These internal electrodes, usually termedaccelerating electrodes, ensure a more uniform potential gradient alongthe length of the tube and hence enable a higher over-all potentialdiiierence to be applied to the tube. Apertures are provided in theaccelerating electrodes for the passage of the accelerated beam.

It is common experience that when such accelerating tubes are operatingat high voltage gradients, unwanted negative currents appear in thetube. These are mainly due to secondary electrons and ions which areformed at the end of the tube which is at negative potential, and areaccelerated towards the positive end by the electro static field. Asthese negative particles also strike the internal tube electrodessecondary emission takes place and the total tube current is thusincreased by a cascade process.

These currents are undesirable in that they present an unwanted load onthe generator producing the high potential across the tube, and thatX-rays are produced when the negative particles strike the internalelectrodes or the positive termination of the tube. X-rays, as well asbeing a potential hazard to the health of the operator of the generator,produce ionisation currents in the dielectric medium surrounding thetube and generator (this is commonly gas at high pressure in the case ofVan de Graaii accelerators). The ionization current, which can be of theorder of a hundred microamps or more, is an additional load on thegenerator, and this, together with the unwanted negative current in thetube, can limit the voltage attainable with machines of limited chargingcurrent capacity such as Van de Graafi generators.

The main object of the invention is to provide an improved arrangementwhich substantially avoids unwanted negative currents and theseconsequent drawbacks.

According to the present invention a charged particle accelerator of thekind comprising annular accelerating electrodes encircling anddistributed along the particle path so as to produce an acceleratingelectrical field, has the electrodes divided into a plurality of groupsarranged end to end, the apertures of each group of acceleratingelectrodes (excepting in some cases the end groups) increasing in sizeprogressively along the direction of movement of negative particles froma minimum at the inlet end to a maximum at the outlet end which isimmediately followed by the minimum sized aperture of the followinggroup whereby stray electrons are trapped.

It will be appreciated that in the case of accelerators for positivelycharged-particles the secondary electrons will move in the reversedirection to the positively charged tes Patent particles whilst in thecase of accelerators for negatively charged particles the electrons willfollow a forward path. It follows, therefore, that in the former case,i.e. a positive particle accelerator, the electrode apertures of eachgroup will reduce progressively in size along the accelerator whereas inthe case of a negative particle accelerator the apertures will increaseprogressively in size. In either case negative electrons will be trappedby the discontinuity in the aperture sizes.

In order that the invention may be more clearly understood referencewill now be made to the accompanying drawings, in which:

FIG. 1 shows diagrammatically an accelerator for positively chargedparticles embodying the present invention, Whilst FIG. 2 is a similarview showing an accelerator for negatively charged particles embodyingthe invention.

FIG. 3 is a perspective view of an accelerator.

In the accelerator for positively charged particles shown in FIG. 1 thereference 1 indicates generally the wall of the accelerator tube. Thiswould consist in general of a series of annular rings 2 of insulatingmaterial, e.g. porcelain, acting as spacers between the plates 3supporting the rings 4 which range in turn support accelerating discelectrodes 5. Graded potentials are applied to the electrodes 5, forexample, as shown in the drawing they are connected to successivetapping points of a resistor chain 6 across which the acceleratingpotential is applied. The reference 7 indicates the source of the beamof positive ions to be accelerated, and 8 is a collector of such ions.

In the arrangement shown in FIG. 1 the accelerating electrodes 5 arearranged in a series of successive groups indicated by the letters A-E.In the first group A the aperture sizes progressively increase, theelectrodes with smaller apertures being necessary at the top in order toshield the low energy positive particle beam at this point from theinfluence of electrostatic charges on the insulating walls of the tube.In the second group B the aperture sizes progressively decrease to aminimum. At the upper end of group C the aperture size is a maximum anddecreases to a minimum at the lower end; similarly in each of the groupsD and E the aperture sizes reduce progressively. Whilst this arrangementwill not have appreciable effect on the positive particles of the mainbeam which are travelling downwards, any stray negative electrons willmove upwards and will be trapped at the junction points between thegroups B, C, D and E where they encounter an accelerating electrodehaving a suddenly reduced diameter.

Although in the arrangement shown protection of the beam from wallcharges near the upper end of the tube is effected by constructing groupA as described above, it should be realised that, if the effect of wallcharges were reduced or eliminated by any other means such as by usingslightly conducting material for the tube walls, it would be preferableaccording to the invention to make group A similar to groups B, C, D andE, thereby providing a further trapping point for stray electrons.

The annular rings 4 serve to prevent the accelerated positive particlestravelling down the central axis of the tube from being deflected byelectrostatic fields due to charges collected on the insulating walls ofthe accelerating tube and do not have an appreciable effect on thereduction of retrograde current.

FIG. 2 shows how the invention would be applied to an accelerator ofnegative charged particles. A beam of such particles would be emitted bythe source 9 and collected on the collector 10. In this case it will beobserved that the potential connected across the resistor chain is, ofcourse, in the opposite sense to that of FIG. 1, and furthermore, theaperture diameters of the electrodes in the groups G, H, I, I, K willincrease progressively downwards since in this case the stray electronswill move downwards and not upwards and hence they will be trapped bythe discontinuities between the successive groups. At the same time themain charged particles of the beam travelling along its axis will not beaifected noticeably. I

In the perspective view shown in FIG. 3 the resistor chain 6 is shown asa stack opposite ends of which are connected to a battery B. This isshown poled for the arrangement of FIG. 1 and would :be reversed forFIG. 2. At the ends of the accelerator are covers 11, 12 respectivelyand these are connected through ducts 13, 14 respectively with anevacuating pump 15.

What we claim is: v

1. An accelerator for charged particles comprising a particle emitter, aparticle collectorspaced from said emitter and annular acceleratingelectrodes encircling the particle path between said emitter and saidcollector, means for applying graded potentials to said acceleratingelectrodes to produce an accelerating field, said electrodes beingdivided into a plurality of groups arranged end to end, at least some ofsaid groups comprising annular electrodes having progressivelyincreasing internal diameters along the direction of negative particleflow including a maximum sized aperture electrode at the downstream endfor said negative particles and adjacent thereto the minimum sizeaperture electrode of the next group downstream Wilth respect to thenegative particle flow.

2. An accelerator for positively charged particles comprising a particleemitter, a particle collector spaced from 4 said emitter and annularaccelerating electrodes encircling the particle path between saidemitter and said collector, means for applying graded potentials to saidaccelerating electrodes to produce an accelerating field, saidelectrodes being divided into a plurality of groups ar-ranged end toend, said groups, excepting the groupat the emitter end, comprisingannular electrodes having progressively increasing internal diametersalong the direction of particle flow between emitter and collector andincluding a minimum sized aperture electrode adjacent the maximum sizeaperture electrode of the next group towards the particle collector;

3. An accelerator for negatively charged particles comprising a particleemitter, a particle collector spaced from said emitter and annularaccelerating electrodes encircling the particle path between saidemitter and said collector, means for applying graded potentials to saidaccelerating electrodes to produce an accelerating field, saidelectrodes being divided into a plurality of groups arranged end to end,said groups, excepting the group at the collector end, comprisingannular electrodes having progressively increasing internal diametersalong the direction of particle flow between emitter and collectorincluding a minimum sized aperture electrode at the downstream endadjacent the maximum size aperture electrode of the nextgroupdownstream.

Trump Sept. 5, 1950 Ginzton Aug. 11, 1959

